Lidar, Laser Sperm Traps Highlight Annual OSA/DLS Meeting

From September 16-20, scientists converged on San Jose, California, for a week of cutting-edge presentations on the latest advances in lasers and optics in San Jose, California at the 2007 Frontiers in Optics conference. This is the annual meeting of the Optical Society of America, as well as the annual meeting of the APS Division of Laser Science (DLS). As such, the conference provides an important forum for the latest work on laser applications and development, spanning a broad range of topics in physics, biology and chemistry.

Near-Infrared Lidar Helps Pilots. Airline pilots will have more advance warning of potentially hazardous atmospheric conditions—such as icing—using a new near-infrared LIght Detection And Ranging (LIDAR) system developed by scientists at RL Associates in Chester, Pennsylvania. The system will also provide better images in foggy, rainy, or extremely hazy conditions, making it easier for pilots to take off and land in those conditions, thereby reducing flight delays. Lidar exploits the same basic principle as radar, using light waves instead of radio waves. It is frequently used in atmospheric physics to measure the densities of various particles in the middle and upper atmospheres.

According to Mary Ludwig, the RL Associates system uses a polarized laser light beam as the source pulse. When the beam encounters aerosol particles in the atmosphere, for example, the light is scattered in all directions. The system then analyzes the backscatter for changes in polarization to determine the nature of the object(s). Other Lidar systems have used similar polarization techniques in the visible spectrum, but the RL Associates system is the first to use near-infrared, which can be operated on runways without damaging pilots’ eyesight.

The system also employs a “range-gated detector” that is only turned on for very short periods of time when the return signal is expected. The camera detector is off when the initial laser pulse is emitted and therefore doesn’t pick up a lot of excess near-field backscatter, usually a large source of noise. So there is a vastly improved signal-to-noise ration, resulting in better images, particularly in obscuring conditions such as fog or haze.

Restoring Sight, One Pixel at a Time. Researchers at the University of Southern California’s Engineering Research Center (ERC) for Biomimetic MicroElectronic Systems (BMES) have developed a tiny camera for retinal prosthetic systems that can be implanted directly into the human eye. It is an important milestone in the ultimate goal of providing limited vision to those rendered blind by certain diseases, via a fully implantable retinal prosthetic device. Current retinal prostheses are designed to be used with an external (extraocular) camera mounted in a pair of glasses.

In order to optimize the design constraints, Tanguay’s group performed a series of psychophysical studies to determine the minimum requirements for the most important characteristics of human visual perception: object recognition, face recognition, navigation, and mobility. They found that very few pixels were required to achieve good results for many of those tasks: 625 pixels in total, compared to more than a million for a typical computer display. They also found that pre-and post-pixellation blurring of images resulted in significantly improved object recognition and tracking– even better for moving objects as with static ones.

Those findings have made it possible to substantially reduce the components of the intraocular camera, thereby reducing the prototype intraocular camera’s size and weight down to about one-third the size of a Tic-Tac. According to USC/BMES team leader Armand Tanguay, Jr., the next generation prototype will be close to fully implantable. One early prototype was successfully implanted into a dog’s eye in July 2004, although human FDA trials are still at least two years in the future.

High-Throughput Sperm Sorting. Researchers at the Irvine and San Diego campuses of the University of California have developed a new high-throughput sorting technique for sperm using a laser trap to separate stronger, faster sperm from slower sperm. Faster sperm are more likely to successfully fertilize an egg, so the technique could improve the chances of conception via in vitro fertilization by ensuring that only the fastest, strongest sperm are used. The technique could find wide application in animal husbandry and human fertility treatments.

UCI scientist Bing Shao and his colleagues used special conic-shaped lenses called “axicons”, which, when combined with a standard lens and a laser, forms a ring-shaped focus (a laser trap). Changing the diameter of the ring makes the trap suitable for imaging cells of various sizes–everything from sperm to algae and microbes. The trap acts as a “speed bump” for swimming sperm, depending on the power of the laser used: slower, weaker sperm below the threshold of the laser power being used will be slowed down, redirected, or stopped altogether in the trap, while faster, stronger sperm are hardly affected at all because their energies are above the critical threshold.

Shao’s new technique could also be used to separate male from female sperm to assist with gender selection. “X sperm generally are heavier and swim slower, while Y sperm are lighter and swim faster,” he explains. “It is certainly possible that this technique can be used for X/Y separation since they swim at different velocities, and might also swim with different forces. As long as the difference is sufficient, we should be able to tell.”

Detecting Malaria with Light. It is possible to analyze large tissue samples for signs of malaria with much greater detail and accuracy, using a macroscope to determine telltale changes in the polarization of light reflecting off the sample, according to the latest research by a team of scientists at the University of Waterloo in Ontario, Canada, and Spain’s University of Murcia. Accurate identification and measurement of population densities of malaria parasites present in a given sample are critical for determining results of clinical trials, according to Melanie Campbell, a researcher at the University of Waterloo and currently president of the Canadian Association of Physicists.

Prior research has demonstrated that the malaria parasite is sensitive to light polarization, and this has been exploited to diagnose blood samples using polarimetry. Campbell and her colleagues have extended this approach to analyzing tissue samples. They used both infected and normal tissue in their experiments, and used a confocal laser scanning macroscope to measure changes in polarization to determine the levels of malaria parasites in the tissue samples.

Using the macroscope means that much larger tissue samples can be imaged at higher resolutions, making it easier to analyze them for signs of the malaria parasite. They also found that they achieved strong contrast of the malaria parasites within the tissue samples (which included retinal vessels), with incident linearly polarized light.

Better Virtual Navigation. Researchers at the University of California, San Diego, have developed a new optical tracking device for improved navigation in a panoramic 3D virtual reality system. Immersive virtual environments are already widely used for surgical and flight training, military training, scientific visualization, and for helping patients with brain injuries recover neuro-motor skills, especially patients recovering from strokes, who are undergoing rehabilitative therapy to regain motor function.

The UCSD system uses five networked computers linked to five large-scale plasma display screens arranged in a pentagon, mounted on a supporting framework. The scene rendered on each display is refreshed in response to the tracking device, which is wireless, so there is no confusing mix of connecting wires when multiple users are involved. Up to five different users can interact with the virtual environment simultaneously with natural motion, just like tasks in the real world.

The main challenge the researchers faced was how to get all five displays and the optical tracking device synchronized, so that a user could perceive the visual and audio feedback on the displays immediately. They overcame this by using a sixth computer devoted just to the tracking via the 3D input device. The sixth computer sends tracking results to all five of the other PCs via a high-speed wireless connection.